1. Field of the Invention
This invention relates to heat-sensitive recording elements useful for making color images, to a method of imaging using said elements and to novel organic compounds useful as the image-forming materials in said heat-sensitive recording elements.
2. Description of the Prior Art
A variety of thermal imaging systems for producing color images have been proposed. In one type of heat-sensitive recording system, a first sheet containing a first reagent is superposed with a second sheet containing a second reagent and one of the reagents is melted or vaporized by the imagewise application of heat and transferred for reaction with the other reagent to form a color image. In another type of "transferring system", images are formed by sequentially transferring two or more dyes carried on separate donor sheets to a common receptor sheet by melting or volatilization. In thermal imaging systems of the "self-containing" type, a single sheet is used and the imagewise heating of the heat-sensitive sheet produces a color image, for example, by rendering a coating layer transparent to reveal the color of a background layer, by initiating the chemical reaction of two or more reagents to form a colored product or by bleaching, coloring or changing the color of a single reagent.
A number of compounds of the latter type, that is, single compounds which undergo a color change upon application of heat have been disclosed. U.S. Pat. No. 3,488,705 discloses thermally unstable organic acid salts of triarylmethane dyes useful in electrophotographic elements as sensitizing dyes that are decomposed and bleached upon heating. U.S. Pat. No. 3,745,009 reissued as U.S. Pat. No. 29,168 and U.S. Pat. No. 3,832,212 disclose heat-sensitive compounds for thermography containing a heterocyclic nitrogen atom substituted with an --OR group, for example, a carbonate group that decolorize by undergoing homolytic or heterolytic cleavage of the nitrogen-oxygen bond upon heating to produce an RO+ ion or RO' radical and a dye base or dye radical which may in part fragment further. U.S. Pat. No. 4,380,629 discloses styryl-like compounds which undergo coloration or bleaching, reversibly or irreversibly via ring-opening and ring-closing in response to activating energies such as light, heat or electric potential, and copending U.S. patent application Ser. No. 646,771 of Alan L. Borror, Ernest W. Ellis and Donald A. McGowan filed Sept. 9, 1984, now U.S. Pat. No. 4,602,263, discloses organic compounds that undergo color formation or color bleaching by an irreversible unimolecular fragmentation of at least one thermally unstable carbamate moiety, for example, triarylmethane compounds including bridged triarylmethane compounds comprising a carbamate moiety, such as, ##STR1##